co.urouxn eves of AirniRorons. 305 



Tlie tliinl i'IimikiiI whicli figures prominontlj ni-ouiid tli(! 

 sensory cells of the ommatiiliuiii is the eipitlielial cell. The cell 

 may eitiier be pigmented or nonpigineiited. In Fig. 11, PI. 

 XXX, a grouj) of four retinula cells is shown partly isolated 

 from one another. Within the interspaces between the adjacent 

 retinula cells wmII be found a number of pigmented epithelial 

 cells, extremely elongated, many of them extending the whole 

 length of the retinula cell. In Fig. 12, PI. XXX, a number of 

 highly attenuated epithelial cells {Ep) is shown closely clinging 

 to the outer surface of the retinula cell. In Figs. 10, 13, 14 aiid 

 17 highly attenuated epithelial cells {EjA, both of the pigmented 

 and the non-pigmented kind, are shown, as they occur either 

 around the ganglion cell or around the retinula cell. 



In no case are these epithelial cells found to establish any 

 organic connection with any part of the sensory cells or of their 

 processes. These attenuated epithelial cells do not differ in any 

 essential way from the ordinary ectodermal cells which are found 

 in the walls of the ommatidium or in the general ectoderm. 



The epithelial cells as they further depart from the periphery of 

 the group of sensory cells, become stouter and thicker. One 

 end of each epithelial cell is usually devoid of pigment granules; 

 this non pigmented end of the cell secretes the chitin which 

 forms the lens-cone. We have already seen that the portion 

 which secretes the chitinous covering, the rhabdomere, of the 

 retinula cell is devoid of pigment granules. In this respect the 

 epithelial cells which form the wall of the pit and the neuro- 

 epithelial cells which form the bottom of the 7)it closely agree 

 with each other. 



A glance at the series of figures (Figs. 17-33, PI. XXXI) 

 will show the degrees of modification undergone by the epithe- 

 lial cells teased out from the different parts of the ommatidium. 

 Those cells (Figs. 17 and 18) which lie at the bottom of the pit 

 undergo the greatest modification. Fig. 17 is the central 

 ganglion cell of the ommatidium, with a few slender epithelial 

 cells around it. Fig. 18 is the retinula cell with a few pigmented 

 epithelial cells clinging to its side. Both differ from the rest of 

 the epithelial cells in their possession of nerve processes. In some 

 epithelial cells, as in Figs. 28 and 25, PI. XXXI, the degree of 

 attenuation is so very great that the protoplasmic character of 



